1. Field of the Invention
The present invention relates to an actuator using a piezoelectric element, and a head-positioning mechanism using the actuator. More in particular, the present invention relates to an actuator using a piezoelectric element, capable of positioning with high accuracy, and a head-positioning mechanism using such an actuator.
2. Description of the Related Art
In recent years, information equipment has increased in precision and demand has risen for an actuator able to operate over a minuscule distance. Actuators for correcting the focal point or controlling the inclination angle of optical systems or head actuators for printers and magnetic disk drives, for example, require the capability of controlling the motion thereof to a very high accuracy.
A magnetic disk drive is one of key devices of in multimedia equipment, the market for which has expanded in recent years. Development of a device having a larger memory capacity is desired for use with multimedia equipment which can handle image and voice data in a greater amount and at higher speed. An increased capacity of a magnetic disk drive is generally realized by increasing the storage capacity of each disk. If the storage density is sharply increased without changing the disk diameter, however, the track pitch is sharply reduced. The resulting technical problem is how to accurately position the head device for reading from and writing onto a recording track. A head actuator having a high positioning accuracy is therefore desired.
With conventional magnetic disk drives, an attempt has been made to improve the rigidity of the movable parts such as the carriage and thus to increase the main resonance frequency of in-plane mode. Nevertheless, increasing the frequency of the resonance frequency has its own limitation. Even if the frequency of the resonance frequency caused by stiffness of carriage arms can be increased remarkably, resonance would be generated by the spring characteristics of the bearing supporting the movable parts. Thus, it is difficult to reduce positioning errors.
Also, it has so far been difficult to increase the servo bandwidth for positioning control of a magnetic disk drive.
A proposal that has been made as a means for solving these problems is to arrange a second actuator for following the track at the forward end of the arm of the head actuator. This second actuator is capable of positioning the head at the forward end of the arm independently of the operation of the head actuator.
Unexamined Patent Publication (Kokai) No. 3-69072 (JP-A-3-69072), for example, discloses a disk drive comprising a sub-actuator, arranged at the forward end of an arm for positioning the head, in addition to a main actuator for a disk drive. This sub-actuator includes two multilayered piezoelectric elements each configured of a plurality of piezoelectric elements in the plane of head movement, each piezoelectric element being adapted to be displaced along the thickness direction. The expansion and contraction of the piezoelectric elements can move the head finely in the same direction as the plane of head movement.
This sub-actuator is formed as a swingable center spring, having the function of a spring, having a thin arm portion with a multilayered piezoelectric element arranged on both sides thereof. This swingable center spring can be configured of a plurality of slits formed alternately inwardly in the direction perpendicular to the longitudinal direction of the thin arm portion. The swingable center spring imparts a pre-load on the multilayered piezoelectric elements and enhances the operation of extension and contraction thereof at the same time. The multilayered piezoelectric elements and the arm are electrically insulated from each other by an insulating material, and lead wires are led from the electrodes at the ends thereof so that a drive voltage for the multilayered piezoelectric elements is supplied through the lead wires.
The problems of the above-mentioned conventional apparatuses are that a multilayered piezoelectric element is difficult to fabricate, that a pre-load mechanism machined with high precision is required and that the electrodes of the multilayered piezoelectric elements are required to be led out by a lead wire or a wiring member. These factors have increased the cost of the sub-actuator.
Further, the invention described in Examined Patent Publication No.2528261 discloses a head-positioning mechanism including a positioning actuator at the forward end of an arm for following the tracks. The head-positioning mechanism is mounted on a coupling plate for coupling an access arm protruded from the carriage of the head actuator to a support spring with a head mounted at the forward end thereof.
The coupling plate on which the head-positioning mechanism is arranged includes a fixed area, a movable area, an extendable area, a hinge section and a gap. A groove is formed on each of the two surfaces of the extendable area. Piezoelectric elements are fixed in the grooves. The piezoelectric elements located at symmetric positions about the center line of the coupling plate are deformed in opposite directions at the same time upon application of different potentials from a power supply to the outer side surfaces thereof, respectively, with the coupling plate as a common electrode.
The head-positioning mechanism having the above-mentioned configuration, however, requires a high accuracy in the fabrication of the multilayered piezoelectric elements and highly accurate outer dimensions of the element. Further, the arm is required to be machined with high dimensional accuracy and the coupling plate having a comparatively high rigidity is required to be extended and contracted. The resulting problem is that strong forces are required to obtain a predetermined stroke in the head-positioning mechanism.